1. Field of the Invention
The invention relates generally to liquid treatment reactors including media beds that are expandable as a result of the growth or accumulation of biological or other material within those beds, and more particularly to methods and apparatus for monitoring or measuring bed size during reactor operation.
2. Reference to Prior Art
Principles of operation of fluid bed biological reactors are provided in U.S. Pat. Nos. 4,182,675, 4,009,105, 4,009,099, 4,009,098, 3,956,129 and 3,846,289. Briefly, a biological fluid bed reactor includes a reactor tank containing a media bed made up of particulate solids, such as sand or activated carbon, with microorganisms (or biomass) adhered thereto to form "bioparticles". A reactor influent, such as wastewater, is conducted upwardly through the reactor tank at a velocity sufficient to fluidize the media bed, and the biomass interacts with that upflow to produce a treated effluent. The interaction between the biomass and the reactor influent also produces biological growth which over time results in media bed expansion that must be controlled to prevent loss of bioparticles with the reactor effluent and to optimize reactor performance.
The phenomenon of biomass growth in fluid bed reactors and the problems associated therewith are well known, and a number of solutions have been proposed for removing excess biomass from the media bed to control bed expansion. For example, in the above-mentioned patents media bed expansion is directly measured as a function of bed height by a bed level sensor, such as an optical sensor for example. When bed height reaches a level indicative of excessive biomass growth the bioparticles are abraded with a mechanical agitator to strip excess biomass therefrom. The stripped biomass is then carried out with the reactor effluent. This is disadvantageous because additional equipment (i.e., sludge dewatering or separation equipment, etc.) may be required downstream of the reactor. Additionally, the sensors employed to directly monitor bed height are subject to fouling and lack the reliability desired for continuous, maintenance-free service.
In U.S. Pat. Nos. 4,177,144 and 4,250,033 carry-over of sheared biomass with the reactor effluent is avoided by confining the sheared biomass to a separator column above the media bed. When activated, an agitator (such as a stirrer or a pump) associated with the separator column generates sheared biomass which is withdrawn from the separator column and partially sheared bioparticles which fall back into the media bed. If the agitator is continuously operated the water content of sludge (i.e., sheared biomass and water) withdrawn from the separator column will likely be sufficient to require dewatering equipment (and its accompanying capital and operating costs). To minimize the water content of the sludge, the agitator is operated only when signaled by an optical sensor or tuning fork device used to measure the height of the media bed. However, as mentioned above, those direct measuring devices are subject to fouling when left in the reactor for extended periods of time, and therefore must be periodically cleaned or only intermittently used (i.e., lowered into the reactor to take a bed height reading and then immediately withdrawn). This requires manual labor and prevents continuous and automatic bed height readings.